Layered silicate materials such as the smectite clays are a class of inorganic particulate materials that occur as stacks of individual, planar silicate layers referred to as platelets in the clay literature. Examples of smectite clays include montmorillonite, bentonite, bidelite, hectorite, saponite, and stevensite. These clays are popular particulate gellants or thickeners for aqueous compositions.
Smectite clays also find use as a flocculation aid in wastewater treatment where the flocculant compositions used are generally solid admixtures of clay and other treatment reagents. These flocculant products, however, are excluded from those wastewater treatment scenarios where the facility is not equipped to handle any solid treatment reagent. Since smectite clay-water slurries turn into gels once the clay content exceeds a level that may be as low as in the range of 5-10% by weight, handling, especially pumping, of clay suspensions with high clay content may prove to be extremely difficult, if not impossible. Nevertheless, in order for it to be viable, any liquid product of clay-based flocculants should have clay content much exceeding the above range. The present invention reveals a method for achieving such desirable liquid products of smectite clay-based flocculants, and the compositions thereof.
The face-surfaces of the platelets of smectite clays bear anionic charges counterbalanced by exchangeable cations that remain electrostatically attracted to the anionic charge of the clay surface. The exchangeable cations are generally either sodium ions or calcium ions. Smectite clay is referred to as sodium or calcium clay, depending on the type of predominant counterions associated with the face-surfaces of the clay platelets. While the anionic charge on the platelet face-surfaces does not vary with pH, the electrical charge on the edge-surfaces of these clays, although anionic under alkaline pH, could be cationic under acidic pH.
Fundamentally, the formation of particulate gels is a manifestation of suspended colloidal particles forming a network structure that entraps and thus immobilizes the suspending medium. Clay-based gels may form when individual platelets or stacks of a few, e.g., 3-15, aggregated platelets (tactoids) engage in interparticle associations with their neighboring platelets. These particle-to-particle links result in a particulate structure pervading through the entire suspension-volume. Such interparticle associations are governed by the interplay between the attractive and repulsive forces that generally act between particles suspended in a liquid.
Clearly, the strength of particulate gels will depend on the number of interparticle associations in a given volume of the gel, implying that the greater the number-concentration of suspended particles, the stronger is the gel. Also, a dominance of the attractive interactions over the repulsive interactions, the likelihood of which increases with decrease in interparticle separation distance, is required for suspended particles to associate with their neighbors. An increase in number-concentration of particles will tend to reduce their separation distances, an effect that could be especially dramatic for planar particles since the separation distance between two adjacent platelets will vary along their lengths when their faces do not align in parallel configuration. Nonetheless, too strong an attraction between adjacent clay platelets may draw them into strong face-to-face association, minimizing the number-concentration of particles.
Considering the above, the key to making clay-based gels is to ensure that there is sufficient interplatelet repulsion for the clay platelets to exfoliate (delaminate) under shear, releasing a large number of platelets as individual platelets or tactoids having fewer stacked platelets, that would then be available to form a particle network. On the other hand, in order to form a voluminous network structure, the net interaction (the sum of attractive and repulsive forces) between the delaminated platelets must be such that they can remain “bound” (attracted) to their neighboring platelets without being drawn into strong face-to-face association. Accordingly, the gel-network may form if the delaminated platelets, while being separated from the surrounding platelets by as thick as possible an intervening layer of the suspending medium, reside in a minimum of free energy of interaction with the neighboring platelets. Albeit physically separated from their neighbors, the individual platelets are no longer free to move independently, being trapped in a free energy minimum, in effect producing a particulate structure, and therefore thickening or gelation. Yet another phenomenon that clay-based gels may form in aqueous compositions, is where clay platelets coagulate due to edge-to-face associations, forming the so-called “card-house” structure described in clay literature.
The sodium smectite clays exfoliate to a much greater extent than their calcium analogs. For this reason, the sodium smectite clays produce a significantly higher level of thickening as compared to the calcium smectite clays. Therefore, one way of having concentrated clay suspensions with high fluidity is to use calcium smectite clays. However, such dispersions, while possibly meeting the requirement of low viscosity, would not present a high number concentration of delaminated platelets, which may be desirable for having good flocculating properties during wastewater treatment.
The ability of clay platelets to bring about coagulation/flocculation of suspended debris particles in wastewater is related to a phenomenon that may be described as heterocoagulation (coagulation between dissimilar materials) between the clay platelets and the debris particles. Such heterocoagulation would occur when the physicochemical conditions of the wastewater are such that the interaction between the debris particles and the clay platelets is attractive, even though the interaction between the debris particles is repulsive, preventing these particles from coagulating. Another way to describe such a heterocoagulation process is to use the analogy of bridging flocculation by polymeric flocculants, as described in colloid literature: like polymeric flocculants, the clay platelets draw the suspended debris particles into flocculation by sticking to and thus bridging two or more debris particles simultaneously.
It may be expected that the aforementioned debris-clay coagulation process will be favored if the number concentration of clay platelets is high (i.e., if the clay platelets are highly delaminated or exfoliated, an effect that also promotes thickening induced by clay platelets) and/or if a strong clay platelet-debris particle attraction is brought into play. Accordingly, conflicting demands are faced in obtaining concentrated, non-viscous clay-suspensions where the clay platelets are sufficiently delaminated in order to have good flocculating power. Nevertheless, even when a large number of clay platelets have been released due to exfoliation, the platelets may be prevented from engaging into any association with the neighboring platelets if the interparticle repulsive forces greatly dominate over the attractive forces. Therefore, the key to attaining concentrated, but non-viscous clay-suspensions, without necessarily sacrificing good exfoliation of clay platelets, is to ensure that strong repulsive forces act between the platelets, superseding any attractive interplatelet forces that tend to bring about associations between the platelets. Most surfaces tend to acquire an anionic charge when wetted with an electrolyte or water. Also, the surface active agents (emulsifiers and dispersing agents) that are more commonly used in industrial applications are anionic, resulting in suspended particles in most wastewater streams that are generally anionic. So in the context of clay-based flocculants, it has been found that a way to increase the clay platelet-debris attraction is to render the surface of the clay platelets cationic through the adsorption of cationic species.
As described in colloid literature, ionic polymers or polyelectrolytes may provide for electrical and steric repulsion forces between suspended particles, if, upon adsorption on the particle surface, i) the adsorbed polymer chains render the particle surface electrically charged, ii) the adsorbed polymer chains occupy more than 50% of the particle surface area, and iii) the polymer segments dangle out off the particle surface into the surrounding dispersion medium, forming loops and tails. Once brought into play, these repulsion forces act to minimize interparticle associations, resulting in thinning of the suspension. The adsorption of cationic polyelectrolytes on the surface of clay platelets could potentially increase the attractive interaction between the clay platelets and the anionic debris particles, which in turn could enhance the flocculating ability of clay.
Although the prior art teaches the use of various types of anionic polymer as thinning agents for smectite clay suspensions, it does not disclose the effects of cationic polymers on the rheological properties (for example, viscosity properties) of concentrated clay suspensions (for example, suspensions having a smectite clay content exceeding 20% by weight). Therefore, it is not clear whether or not the addition of a cationic polymer to a concentrated suspension of smectite clay would produce either a “thin” or a highly shear-thinning suspension that shows a viscosity significantly lower than what it would have been in the absence of the polymer. It is only since the present invention that it has been found that cationic polymers with a weight average molecular weight falling within a certain range, when used even at relatively low concentrations, would render a concentrated suspension of smectite clays non-viscous, while the suspension shows considerable flocculating ability.
Although a targeted application for the product of the present invention is coagulation/flocculation of suspended matter in water streams as, for example, in wastewater treatment, because of the coagulating ability of the product, it may be used even as a drainage aid in the papermaking process, wherein the product helps in bringing about agglomeration of pulp fibers to facilitate the drainage process.
Other potential uses of the product include, but are not limited to, an additive in personal care and cosmetic formulations, as well as a fabric softener. The cationic polymer-modified clays would be substantive to (or adhere onto) the anionic surfaces of the hair or the skin, such that these clays can help deliver some useful hair care or skin care properties when used as an additive in personal care or cosmetic products. For example, the deposition of cationic clay platelets on the anionic surface of the hair shafts is expected to be substantive to hair shafts to enhance hair styling. Accordingly, the use of cationic polymer-modified clays in hair care products should add to the hair styling properties of these products. The prior art discloses the use of smectite clays as a fabric softener. Furthermore, the most commonly used fabric softeners are cationic surfactants. As for the molecular structure, surfactant molecules contain a hydrophilic part and a hydrophobic part, with the two parts of the molecule segregated from one another. The cationic surfactants used as fabric softeners impart softness by adsorbing onto the (negatively charged) fabrics with their hydrophilic part, consisting of the cationic functional group, attached onto the fabric surface, while their hydrophobic part projects outwardly from the surface. Such adsorption of the cationic surfactants minimizes interfacial tension between the fabric surface and the surrounding air mass and minimizes the adhesion of water to the fabric surface. This reduces the shrinkage (reduction of substrate surface area) and resulting hard “feel” that accompanies the removal of water from the substrate. In accordance with the present invention, the cationic modification of the smectite clay surface by the surface treatment of the clay with one or more cationic species, e.g., polymers, that contain one or more hydrophobic groups, render the clay platelets better equipped to serve as a fabric softener.